U.S. Pat. No. 6,024,792 shows the build-up welding used for single crystal structures, in which new layers are produced on a substrate by means of powder being supplied.
Turbines are used in many sectors to drive generators or turbo machines. The energy content of a working medium which is under a high pressure and at a high temperature is used to generate a rotary motion of a turbine shaft by the working medium being passed over the turbine, where it expands in such a manner as to perform work. To generate the rotary motion of the turbine shaft, a number of rotor blades, which are usually combined in ring form to form groups or rows of blades, are arranged over the outer circumference of the shaft, which rotor blades drive the turbine shaft by transfer of momentum from the working medium flowing in. Moreover, rows of guide vanes which are connected to the turbine casing are usually arranged between the adjacent rows of rotor blades in order to guide the flowing working medium in the turbine. The turbine blades and vanes have a main blade or vane section which is profiled in a manner which is advantageous for appropriate guidance of the working medium, is configured appropriately for the working medium parameters, such as pressure and temperature, and extends along a blade or vane axis. A platform, which extends transversely with respect to the blade or vane axis and is formed as a hook pedestal at least in an end region, is formed integrally on the end side of the main blade or vane part in order to attach the turbine blade or vane to its corresponding support, i.e. the shaft or casing.
To achieve a particularly high level of efficiency, gas or steam turbines for driving generators are for thermodynamic reasons designed for particularly high parameters of the working medium flowing into the turbine unit. In the case of steam turbines, this may quite easily mean pressures of over 200 bar and temperatures of over 500° C.
In the case of gas turbines, the temperatures of the working medium flowing into the turbine unit may lie in the range from approximately 1200° C. to approximately 1300° C.
To ensure a high level of reliability and a long service life of a turbine component, in particular a blade or vane, even at very high working medium parameters and correspondingly high thermal loads, turbine components which are exposed to particularly high levels of loading are designed to be coolable.
Furthermore, rotating turbine components are exposed to particularly high mechanical loads in the radial direction. For example, turbine blades or vanes generally have their axis orientated in the radial direction and have to withstand considerable tensile stresses in this direction. The mechanical loads are increased still further since, in modern turbines, the turbine blades or vanes are usually designed as what are known as hollow profiles, in order to ensure cooling of a turbine blade or vane in the inner region by acting on cavities in regions which are exposed to particularly high thermal loads. It has proven expedient to produce a turbine component from a material which has a preferred crystal orientation. For this purpose, the turbine component may be designed as a single crystal component, i.e. the preferred crystal orientation is given by the single crystal structure. A component of this type is also known as an “SX component”. Alternatively, however, the preferred crystal orientation may also be produced by the turbine component being designed with a directionally solidified structure. A component of this type is also known as a “DS component”. The preferred crystal orientation of the material of the turbine component makes it possible to achieve a particularly good, specifically directed and rapid, dissipation of heat and an ability to withstand tensile loads along the preferred crystal orientation which is considerably higher than with a turbine component made from a material which is isotropic. In the case of a turbine blade or vane, the preferred crystal orientation is expediently arranged along the blade or vane axis. The heat conduction properties of a turbine blade or vane of this type and its ability to withstand tensile loads are therefore particularly expedient along this preferred crystal orientation for operation of the turbine blade or vane in a turbine. Similar measures can also be provided in a similar way for other turbine components.
Turbine components which are exposed to considerable mechanical and thermal loads, in particular turbine blades or vanes, are usually produced entirely or in part, as described in EP 0 815 993 A1, using a casting process which enables the turbine component to be provided with a directionally solidified and/or single crystal structure as a preferred crystal orientation. Therefore, SX or DS blades or vanes are usually cast in a single operation. The casting process means that undercuts, as are used in particular in the case of the turbine blades or vanes with an anchoring platform or cavities as explained above, cannot be achieved or can only be achieved with difficulty. In any event, complex geometries generally lead to high scrap rates, which reduces the economic viability of the casting operation, which has to produce a turbine component of this type as a single component. Hitherto, these high scrap rates have been accepted. One attempt to lower scrap rates in a turbine component with a preferred crystal orientation, in particular in an SX and/or DS turbine component, consists in designing a turbine component of this type such that it is “suitable for casting”, i.e. generally with simpler design than was originally desired. This is often to the detriment of the use properties of a turbine component of this type.
To summarize, therefore, it can be concluded that turbine components, in particular rotating components, which are exposed to high thermal and mechanical loads and at the same time have a design which is complex with regard to the casting operation can only be produced with difficulty with a preferred crystal orientation in the casting process. Either high scrap rates have to be accepted or a turbine component design which has been unnecessarily simplified purely for the casting operation has to be accepted. These problems are all the more serious since hitherto a single-crystal or directionally solidified turbine component has had to be cast as a single part using a casting process.